It is a well recognized practice to apply some sort of acoustic absorber material at the ends of the piezoelectric surface and also on certain portions of the metal transducers as well as the multi-strip coupler of a surface acoustic wave (SAW) filter, by way of example. By absorbing undesired modes of the SAW, both the frequency response of the filter as well as side lobes and the rejection of various time spurious responses such as edge reflections may be significantly enhanced.
Typically, SAW device manufacturers use a family of rubber materials known as room temperature vulcanizing (RTV) epoxies or resins applied in either manual or automatic fashion for the suppression of unwanted acoustic energy, such acoustic waves that reflect off chip edges. This type of material has been proven to be very effective for such application due to its high absorption qualities and stable temperature performance capabilities. One major drawback of RTV is that the method of application wherein material is typically forced out through a syringe is very coarse, thus not allowing designers to lay out a precise or specific acoustically desirable pattern. Another method for a high volume and low cost technique of applying an acoustic absorber to SAW devices is the use of a solder mask printed circuit board (PCB) screen printing technique. The absorber material is similar to the one used for solder mask for PCB screen printing.
By way of example, FIG. 1a illustrates one screened absorber SAW device having input and output transducers and a multistrip coupler for coupling energy between the transducers The absorber material is applied using a stainless steel screen mask used to “print” the desired absorber pattern just before the cutting of wafers. This steel mask becomes part of the design process and therefore the shape of its pattern needs to be precise and contingent on each filter specification. Also, it has the additional advantage of allowing accurate measurement of the filter response at the wafer probe level. However, this method has several disadvantages. By way of example, the absorber material “bleeds out” as illustrated with reference to FIGS. 1b and 1c. The material tends to expand after curing by 100 to 160 microns in all directions along the surface. Therefore, the active “real estate area” is effectively reduced for the active transducers. As a result, it becomes very difficult to design high performance intermediate frequency (IF) filters on packages smaller than 5×5 mm. Further, the performance of this absorber material at cold temperature is inadequate for several filters with low error vector magnitude (EVM) as has been reported by device users and design engineers. The lack of performance is due to the fact that this material becomes stiff and rigid at cold temperature. It reflects, instead of dampening, the waves back to the transducers thus distorting the pass band filter response and degrading its EVM.
Yet further, the typical absorber application method is not compatible with a standard semiconductor photolithography process. The current screen printing method increases overall wafer level manufacturing costs because of the requirement that an operator be employed to manually align the absorber mask with the patterned wafers. In addition, the normal process is disrupts the normal process flow which needs to be streamlined to avoid all possible manufacturing mistakes.
Typical methods making use of polymers that can be photo-definable suffer from poor absorption of acoustic wave energy and thus have limited usage. As the typical polymer thickness achievable is several microns, the use of polymers will generally require a multilayered fabrication process.
The present invention provides a measured improvement in performance of an epoxy based photo definable acoustic absorber when compared to current screen printing processes and provides desirable advantages over well known photo definable methods and RTV type materials. The present invention desirably eliminates or mitigates the above described drawbacks and retains excellent absorption qualities when compared to RTV.